1. Field of the Invention
This invention relates generally to a device and a method for optically inspecting a surface.
2. Description of the Related Art
Image recording devices for imaging defects on a planar, reflective surface are known in the art. For example, U.S. Pat. No. 6,870,949 discloses such a device where the section of a surface to be investigated is imaged by a camera, employing a telecentric beam path, and the dark-field illumination of the surface is provided by an annular light source arranged coaxially with respect to the camera's optical axis. It also provides that light from the light source employed for illuminating the surface is normally reflected to the camera's optical axis by a semitransparent mirror, in which case, a bright-field illumination of the surface might also be implemented. The image of the surface to be investigated is thus imaged onto the camera's optics by the field lens and the beam path transits the plane-parallel substrate of the semitransparent mirror, which in turn will likely create image-degrading astigmatism, particularly in cases where the beam has a large divergence angle, such as where the field lens has a short focal length.
U.S. Pat. No. 4,561,722 discloses a beamsplitter for an illuminator-and-sensor-side telecentric beam path that provides for at least a transparent masking of the semitransparent mirror in order to prevent stray-light effects due to the presence of contamination.
Japanese Published Patent Application JP08172506A discloses a manuscript reading device having a camera and nearly on-axis, dark-field illumination, where the same telecentric optics are employed for illumination and configuring the camera's beam path. The nearly on-axis, dark-field illumination yields benefits in terms of the better legibilities of characters, particularly in the case of manuscripts printed on glossy paper.
U.S. Pat. No. 6,870,949 and Japanese Published Patent Application JP 08172506A disclose arrangements involving a camera whose lens lies on the optical axis of the telecentric field lens. These arrangements have the disadvantage that the overall heights of the arrangements, measured along the optical axis of the field lens, increase the focal length of the field lens by an amount equaling the overall length of the camera and its lens.
German Patent DE2354141A1 discloses a device and a method for investigating the planarities of reflective surfaces, where a single point light source is arranged on the optical axis of a field lens. A camera creates an image from which the inclination of the normal vector to the surface to be investigated relative to the optical axis of the field lens is derived. In cases where textures and/or symbols on a reflective surface are to be measured, the device having a point light source has the disadvantage that the illumination will be highly inhomogeneously distributed over the surveyed field for extremely slight inclinations of the optical axis of the path of the illuminating beam relative to the normal to the surface to be investigated, and detection of the textures and/or symbols involved will be severely impaired.
U.S. Pat. No. 4,241,389A discloses a device that creates a number of virtual light sources in the focal plane of a telecentric beam path for the purpose of illuminating a projection mask, employing the Köhler method.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.